As examples of semiconductor optical device, there are optical devices that perform electrical-to-optical conversion and optical-to-electrical conversion, such as light emitting devices and light receiving devices, and there are optical devices that transmit optical signals, such as optical waveguides, optical switches, isolators, and photonic crystals. Such an optical device has a semiconductor laminated structure as a main structure, and a method of manufacturing the optical device may include a thermal treatment process for changing the physical properties of a predetermined semiconductor layer in the semiconductor laminated structure.
For example, in a GaAs-based semiconductor laser, a light emitting end face may be deteriorated due to high light density, which causes a so-called COD (Catastrophic Optical Damage). In order to solve this problem, a method has been proposed which allows a region corresponding to a light emitting surface to have a greater bandgap than a bandgap in an active layer, thereby providing a window region which absorbs less laser light than the inside of the active layer. The window region is formed through the following process. A dielectric film that accelerates the diffusion of Ga is formed in the window region and a dielectric film that suppresses the diffusion of Ga is deposited in a non-window region. After that, a predetermined thermal treatment is performed to cause mixture in a region corresponding to the window region, thereby increasing the bandgap. This method is called an Impurity Free Vacancy Disordering (IFVD) method (see Patent Literature 1). In the process, a SiNx film which is formed under a N-rich condition is used as the dielectric film which accelerates the diffusion of Ga and a SiNx film which is formed under a Si-rich condition is used as the dielectric film which suppresses the diffusion of Ga (see Patent Literature 2).